1. Field of the Invention
This invention relates generally to methods for producing integrated circuits, and more particularly to methods for producing very large scale integration (VLSI) integrated circuits.
2. Description of the Prior Art
Integrated circuits can have hundreds, thousands, or even millions of active and passive components formed on a common, semiconductor substrate. Usually, adjacent components must be electrically isolated from each other to avoid shorts through the substrate which would impair the circuit's functioning. This inter-component isolation typically takes the form of "field oxide" regions comprised of relatively thick layers of silicon dioxide (SiO.sub.2).
Isolation in large scale integration (LSI) and very large scale integration (VLSI) circuits is most often accomplished by a process known as local oxidation (LOCOS). With the LOCOS process, a thick layer of oxide is thermally grown over the surface of a semiconductor substrate, and then a thick layer of silicon-nitride (Si.sub.3 N.sub.4) is deposited over the oxide layer. The nitride and oxide layers are patterned by conventional techniques, and field oxide is grown over and into the exposed surfaces of the substrate.
The LOCOS process has many advantages. For one, it is a relatively simple, and is well-adapted to mass production techniques. In addition, the LOCOS process is advantageous in that it provides self-aligned field implants.
For a variety of reasons, the LOCOS process cannot presently be used when the individual components have dimensions smaller than about one micron. One of the problems with using the LOCOS process in an attempt to produce sub-micron components is the so-called "bird's beak" effect where the field oxide diffuses into the oxide layers covering the active regions of the component in a pattern that looks, in cross-section, like a bird's beak. This diffusion pinches off the active region of the component and impairs or destroys its proper functioning.
Another problem with using the LOCOS process in an attempt to produce sub-micron components is that the resultant surface topography is not sufficiently planar for subsequent VLSI lithography and anisotropic etching steps. This problem is discussed in "Isolation Technology for Scaled MOS VLSI", by W. G. Oldham, IEDM Technical Digest, pp. 216-219, December 1982.
A process known as side wall mask isolation (SWAMI) has been suggested to partially solve the bird's beak problem. As described in "The SWAMI--A Defect free and Near-Zero Bird's Beak Local Oxidation Process and its Application in VLSI Technology", by K. Y. Chiu, et al., IEDM Technical Digest, pp. 224-227, December 1982, the SWAMI process involves silicon etching and the formation of nitride on the sidewalls of the field oxide to prevent diffusion of the field oxide into the oxide layers covering the active regions of the integrated circuit components. By using SWAMI techniques, component sizes can be shrunk to approximately 0.8 microns.
However, the SWAMI process is relatively more complex than the more standard LOCOS process, which makes integrated circuits made by the SWAMI process relatively more expensive. Furthermore, it has been found that for isolation spacing below one micron, the field oxide thickness is a direct function of the opening size in the silicon substrate. Since the silicon openings with the SWAMI process are quite small due to the nitride applied to the side wall areas, the field oxide tends to be extremely thin, and may not provide adequate component isolation. An article discussing this problem is "Scaling Limitations of Sub-Micron Local Oxidation Technology", by J. Hui, et al., IEDM Technical Digest, pp. 392-395, December 1985.
A process known as sealed interface local oxidation (SILO) is described in "Electrical Properties of MOS Devices Made With SILO Technology" by J. Hui. et al., IEDM Technical Digest, pp. 220-223, December 1982. The SILO process attempts to solve the bird's beak problem by providing a blocking structure including a thin nitride layer formed over the components' active regions, a thick oxide layer formed over the thin nitride layer, and a thick nitride layer formed over the thick oxide layer. The thin nitride layer prevents the diffusion of oxygen into the thick oxide layer, and thus substantially eliminates the bird's beak effect.
Unfortunately, the thin nitride layer tends to cause crystal defects in the active regions of the components, and is hard to remove from the silicon substrate for subsequent processing. Furthermore, the field oxide grown by the SILO process creates an uneven surface topology which, as was explained previously, is a poor surface for subsequent VLSI lithography and anisotropic etching steps.